1. Field of the Invention
The present invention relates to phase modulation, and in particular to reducing amplitude distortion during in-phase quadrature (I-Q) phase modulation.
2. State of the Art
Wireless communication systems generally rely on phase-based modulation formats such as Gaussian minimum-shift keying (GMSK). One technique for performing phase modulation in these systems is by phase modulating a voltage controlled oscillator (VCO) using dual port tuning/modulation. However, this approach tends to be inaccurate and has several disadvantages such as it requires a fairly critical determination of the output VCO loop bandwidth in an effort to suppress phase noise and spurious signals while minimizing the effects of frequency pulling. An alternative phase modulation technique is quadrature modulation.
Quadrature modulation is a modulation technique in which separate in-phase (I) and quadrature (Q) signals are used to perform the modulation. FIG. 1 shows a prior art design of a quadrature modulation system which includes a phase mapper 10 generating a quadrature pulse stream which includes an in-phase (I) component of a pulse stream 10A and a quadrature (Q) component of a pulse stream 10B together representative of an input digital information signal. The I and Q pulse streams are coupled to a pulse shaping block 11 which converts pulse streams 10A and 10B into band limited I and Q modulation component signals 11A and 11B (i.e., in-phase component=Sin(xcfx86) and quadrature component=Cos(xcfx86)). The I and Q modulation component signals, 11A and 11B, are coupled to mixer block 12 along with I and Q carrier signals (Cos(xcfx89ct+xcex8) and Sin(xcfx89ct+xcex8), respectively) so as to superimpose each of the quadrature modulation component I and Q signals 11A and 11B onto a carrier signal to generate analog signal 12A, Q(t)xc2x7Sin(xcfx89ct+xcex8), and analog signal 12B, I(t)xc2x7Cos(xcfx89ct+xcex8). Signals 12A and 12B are summed by summer 13 to generate a single phase modulated RF signal s(t)≈Cos(xcfx89ct+xcex8+xcex8)((13A, FIG. 1), where   φ  =            tan              -        1              ⁢                  Q        I            .      
Ideally, for pure phase modulation, the output signal 13A is a constant-envelope signal (i.e., modulated signal with constant amplitude). However, due to gain and offset errors which are inherently caused by conventional quadrature modulator circuit imperfections, output signal 13A exhibits signal distortion components that are comparable in amplitude to the amplitude response at the frequency of interest. This type of distortion can be detrimental to system performance, and, more particularly, can result in the transmission of erroneous signals. In particular, in cases in which output signal 13A is subsequently applied to a tracking (or translation) phase-locked loop, such as in a cellular phone handset, the spurious signals from the quadrature modulator can be amplified, rather than suppressed.
There are many manners in which to reduce spurious output signals from a quadrature modulator due to practical circuit imperfections, namely, the matching of circuit balance, elimination of circuit offsets, and quadrature accuracy improvement, as described in xe2x80x9cTaming the Quadrature Modulatorxe2x80x9d by E. McCune, Proceedings of the Wireless Symposium 2000 (Feb. 22-25, 2000), San Jose, Calif. These techniques are cumbersome, time-consuming, and expensive. Thus it is generally accepted that the nearly universal use of quadrature modulators in digital cellular handsets in high-volume manufacturing naturally involves spurious signals due to these practical circuit imperfections.
What is needed is a system and method in which to use quadrature modulation so as to generate a phase-modulated signal having reduced distortion.
A system and method of modulation is described which generates a modulated signal having an intended modulation scale and having reduced spurious signals. According to one embodiment of the method, initially, an information signal is quadrature-modulated at a modulation scale that is a factor of N times that of the intended modulation scale (referred to herein as quadrature over-modulation). The resulting quadrature over-modulated signal is then frequency divided by the same factor of N. The net effect of the over-modulation and subsequent frequency division is that spurious signals that are created during the quadrature over-modulation process are logarithmically reduced by a factor of 20 log10(N)dB, thereby significantly reducing spurious signals seen within the resulting modulated signal.
In accordance with another embodiment of the method of modulation, quadrature over-modulation is performed on an information signal to generate an over-modulated phase-modulated signal including I and Q continuous signals representing a modulation scale that is N times an intended modulation scale. Each of the I and Q signals are then mixed with carrier signals having a frequency that is N times an intended carrier frequency and are summed to create a single, constant-envelope over-modulated signal. The over-modulated signal is then converted into a digital signal and is frequency divided by a factor of N to generate a modulated signal having the intended modulation scale and the intended carrier frequency.
One embodiment of the system of modulation includes a quadrature over-modulator for generating a continuous over-modulated signal including I and Q component signals representative of the input information signal where the modulation scale representing the I and Q component signals is N times an intended modulation scale, a mixer for mixing each of the I and Q component signals onto a carrier having a frequency that is N times the intended carrier frequency to generate I and Q over-modulated RF signals, a summer for summing the I and Q over-modulated signals to generate a single, constant-envelope over-modulated signal, a converter (such as a limiter) for converting the over-modulated signal into a digital signal, and a digital frequency divider for dividing the frequency of the digital signal by N to generate a modulated signal having the intended modulation scale and the intended carrier frequency.